So, I've got two young eight-inch koi in a 180 gallon tank right now. They'll be moving out to a larger pond (they outgrew the first one) once we manage to get it set up. In a previous tank, I let cladophora algae grow on rocks and driftwood so they could graze on that, and regularly sacrificed hornwort, duckweed, etc. to them. They're voracious. I had once read that java fern was unattractive to them, but they disproved it by ripping it to shreds. We seem to have stupidly high nitrate levels straight out of the tap, so I always do try to keep planted aquariums. My two pretty monsters won't let me get away with that anymore. Right now, I'm hanging aquaponics baskets into the tank and giving them more sacrificial plants to take care of this. In the meantime, I'm wondering whether I can find something they'll leave alone. My water parameters are quite good, but it's a lot of work. I'm currently considering some really odd options: a small mangrove or two, or possibly papyrus. Does anyone have suggestions for a koi-proof plant until I can get them outside? Many thanks!